语法与词汇 It takes at least five years to ten years_____ it is possible to test this medicine on human patients.
【正确答案】 A
【答案解析】[解析] 本题考查时间状语从句。本句是before引导的时间状语从句的固定句型：it+谓语+some time+before sb. do (did) sth. ，只有A项before放入句中符合。